Model Engineers’ Workshop – August 2019

24 http://www.model-engineer.co.uk Model Engineers’ Workshop

a complete revelation to be honest! The
setting dimensions usually involve decimal
millimetres, but the Penguin dial graduations
cater easily for this, and as the central gash
is wide enough there is room for slackness
here. I fi nd that it is easy to get 3 or 4 end
sharpening sessions before the geometry
of the cutter becomes compromised and
then it becomes necessary to deal with the
secondary clearance.
Grinding the spiral edge of the cutter
is a lot more diffi cult at fi rst and requires
further modifi cation to the machine.
When grinding the periphery of the
cutter, we will be using the face of the wheel
with the grinding rest mounted above the
cutter. Grinding on the edge of the wheel
is impossible as we don’t have any means
of elevating the grinding wheel relative to
the cutter centre line, the normal method of
obtaining the clearance angle. The cutter is

angled upwards and there is little clearance
for the grinding rest, see photo 27 later.
This is the only position where the
mounting for the rest which is above the
work holder can be positioned, but there is
a problem. The cutter will be forced away
from the rest by the downward rotation of
the wheel and consequential rotation of
the cutter without an extreme bit of luck
and care will be forced downwards thus
reducing clearance to zero and rendering the
cutter useless. The only solution is to have
the wheel rotating upwards and forcing the
cutter onto the rest and not away from it.
As we will see later this makes for very easy
setting of the height of the rest to obtain the
correct clearance angles
I fi tted a toggle switch into where the
old now unused bar stop was positioned
using a 10 amp spade switch, but this was
not as easy as it seems. With the machine
on its back, the base is removed, and the
redundant old lever fi xed to the front bar
(which controlled the now unused adjuster)
can be seen. This must be removed, to
give space both physical and for electrical
safety purposes. It is achieved by drilling a
few shallow holes into it then splitting the
casting with a chisel, photo 23. Four holes at
the corner of a square were drilled through
from the front of the machine. The waste
was cut out starting in the existing M8 hole

with the jig saw, a surprisingly easy task with

a good quality metal blade, photo 24.

A thin plate of yellow plastic chosen to match

the main switch is drilled for the switch and

fi red to the machine with 4Ba screws.

In order to reverse the motor, all that is

required is to switch over the mains feed

to the running windings, but this must be

done safely. On the 6 pin terminal block

on the motor there was already one spare

connection, and a little rearrangement

of the others gave a second spare. These

provided the connections to the reversing

switch. One of the links had to be removed,

photo 25, fi g. 6.

Unfortunately, the before image of the

connections was lost. The connections

shown relate to my machine, yours may be

diff erent, although the principal remains

the same, take care and double check

before switching on. Close up of the fi nger position

Use spade crimp fi tted connectors

which are reliable and give you the chance

of changing the connections if the motor

rotation does not match the switch

position. If you don’t have a decent pair of

crimping pliers, remove the plastic shield on

the spade fi tting, solder the wire in using

heat shrink tube to provide the insulation

protection. I wanted a down movement

of the switch for down (anticlockwise)

rotation, which is the normal position, and

switched up for clockwise up rotation. The

switch is provided with a missile type switch

cover which automatically sets the switch

to standard down rotation when the cover

is down. You will need two female spade

connectors which have an additional male

blade position, see photo 26.

Calculating the clearance angle

The tool rest needs to be set above the

cutter centreline by an exact amount

dependant on both the required clearance

angle and the diameter of the cutter the

table shows what is needed but the maths

for calculation is given in table 1 which

shows the grinding fi nger rest setting for

diff erent cutter diameters.

Clearance = Sin (angle) X cutter radius

Grinding is achieved by rotating the cutter
counter clockwise against the fi nger rest
starting this in the root fl ute and pulling
towards you at the same time.

End of the cut. Aft er doing the fi rst edge,

switch off , note the feed setting on the main

bar then back off , rotate the cutter, to the next

edge, switch on, add cut and so on. Because of

the geometry of the machine, the grinding will

be extended further than the original but will

not have fl ute which is not a problem.

The now sharp cutter seen earlier the slot

was completed with a single cut while the

end two, at full depth, which would not have

been possible before as seen in photo 17!

Photographs 27 to 32 with their

captions show the grinding procedure.

While cutters less than 6mm can be

sharpened with diffi culty, they should

be regarded as throw away. This is where

the Quorn with its spiralling hobs has the

real advantage, but let’s face it, Professor

Chaddock designed the machine for this

very purpose - making very small cutters.

Finally, photo 34 shows what the cutter

from photo 17 can do. ■

Cutter Diameter

6

9

12

16

20

22

Setting for 6

degree

Clearance in

thousandths of

an inch

13

20

26

33

39

46

Table 1

30

31 32 33